[Date Prev][Date Next] [Thread Prev][Thread Next] [Date Index] [Thread Index]

Re: GPT and SSDs

On 19/05/14 09:04 AM, Henrique de Moraes Holschuh wrote:

On Mon, 19 May 2014, Gary Dale wrote:

This led me into partitioning. I thought that GPT pretty much gave
you proper partition alignment, but now I think I was mistaken on

Unfortunately, it doesn't :-(


that point. gdisk shows my first partition starting at 34, with the
fdisk info showing 255 heads, 63 sectors/track. That doesn't come
close to 4k alignment.

You're using a SSD.  You need 4KiB aligment, but you really want erase-block
alignment.  When in doubt, use the 1MiB default alignment used by Debian
(and most other Linux distros) and also by MS Windows.

Since Debian Wheezy, the Debian installer for x86 (i386 and amd64) should
have done this properly as long as you let it wipe any preexisting partition
tables (that might be incorrectly aligned).

Unfortunately, the Debian installer for other arches (such as powerpc) may
be buggy and get it wrong (in fact, powerpc's installer fails to align GPT
partitions).

BTW: you also need to align the data area of any LVM PVs (which really can
only be done at PV creation time), and create the filesystem with the proper
alignment for their internal structures.  The debian installer for i386 and
amd64 is known to do this correctly, but YMMV on any of the other arches.


Before I haul out sysrescuecd and move my partitions around, I
thought I'd ask for suggestions. Should I move the filesystem to
something that is a multiple of 8, do something else, or look
elsewhere for a solution to my problem?

Align everything to 1MiB (or, if you happen to know it, the erase block size
of your SSD, which will be a submultiple of 1MiB).

You can ignore cylinder alignment: the BIOS would boot using LBA information
anyway and ignore the cylinder/sector/head crap, EFI will grok the GPT
natively, and you should not be letting any operating systems old enought to
not operate in LBA mode anywere close to a raw device.

Don't use half-assed tools to mess with the partitions.  Proper tools known
to get aligment right by default on Debian stable/testing are: gdisk,
cfdisk, mdadm, lvm, mke2fs (mkfs.ext4), mkfs.xfs (for XFS).  "parted" gets
it right only if you give it fuzzy units (such as "0%" instead of "0" for
the start of a partition).  fdisk also aligns correctly when it is not
trying to be compatible with old DOS.
When I use gparted with 1M alignment, it didn't change the 34 start sector. In fact the only change was that gdisk now reports 2-sector alignment: 

  Found valid GPT with protective MBR; using GPT.
  Disk /dev/sda: 234441648 sectors, 111.8 GiB
  Logical sector size: 512 bytes
  Disk identifier (GUID): 8017C70A-FA69-48DD-9227-825774B31816
  Partition table holds up to 128 entries
  First usable sector is 34, last usable sector is 234441614
  Partitions will be aligned on 2-sector boundaries
  Total free space is 911 sectors (455.5 KiB)

  Number  Start (sector)    End (sector)  Size       Code  Name
     1              34       232343551   110.8 GiB   8300  Linux filesystem
     2       232343552       234440703   1024.0 MiB  EF02
I then tried it by putting 1M free in front of the first partition. This changed the start sector to 4096, which is definitely on a 4k boundary, if slightly wasteful. This confirms your point that parted is somewhat flaky when dealing with alignments.
Anyway, with this new alignment, my computer doesn't lock up for as long. I'm still getting a few seconds here and there however. I have a network switch that may be going bad - my brother in law couldn't get it to work with his laptop, so maybe there's problem with it too.
Reply to: